This invention relates to a system combining two vessels for handling liquid natural gas and other natural petroleum products, with one vessel being a floating production, storage and offloading lying at a petroleum producing seabed arrangement, ant with the other vessel being an LNG tank vessel or an ordinary tanker.
In connection with and during offshore production of floating gas (LNG, NGL and LPG) usually stabilized oil and/or condensate is produced. Delivery of LNG is usually tied to long-term contracts, while gas condensate contains light or xe2x80x9cvolatilexe2x80x9d petroleum components which may be delivered at a spot marked to the refinery which at any time might need such light petroleum components for the refining process. The vessels devices for the export system from such a combined offshore oil/gas conversion FPSO must satisfy both these products, export demands in a way so that two different tanker vessels like LNG tank vessels and conventional tank vessels shall be able to moorage to the FPSO-vessel and be connected with their respective transfer systems, whereof the first is cryogenic for LNG transfer, and the second may be arranged for transfer of petroleum components being fluid at higher temperatures.
Liquid Natural Gas xe2x80x9cLNGxe2x80x9d is a liquid, fluid methane, with boiling point between xe2x88x92165xc2x0 C. and xe2x88x92163xc2x0 C.
Gas condensate is the part of the gas from a producing well which is separated into liquid phase, consisting of light petroleum components.
The term xe2x80x9ccryogenicxe2x80x9d describes in this relation a thermally isolated system arranged for handling a gas being cooled down to its liquid phase. The liquid here is LNG. The isolation usually consists of vacuum combined with ordinary isolating material.
By xe2x80x9cLNG pipexe2x80x9d pipes being cryogenic are meant, and may comprise pipes with several parallel channels or concentric channels, provided with isolation and possible return channels arranged outside of the main channel. LNG pipes may be stiff or flexible as defined below.
By xe2x80x9crigid LNG pipesxe2x80x9d pipes are meant which are no more flexible than the occurring change of shape due to pressure or temperature expansion during use.
With the term xe2x80x9cflexible LNG pipexe2x80x9d an LNG pipe is meant being arranged for and which repeatedly may be bent to a smallest radius of bending of e.g. 3 metres. Such flexible LNG pipes are usually provided with corrugated walls of austenitic steel.
An LNG tank vessel is a tank vessel with cryogenic tanks, usually spherically shaped, arranged for transporting LNG.
xe2x80x9cSTPxe2x80x9d and xe2x80x9cSTLxe2x80x9d are submerged moorage- or production loading buoys.
Methane arrives in gas phase from a gas producing petroleum well and must be condensed in a condensing plant to be brought into liquid phase as LNG. LNG has little volume with respect to the methane gas, and may be handled under low pressure. All heat energy supplied to LNG may lead to boiling and thus loss of methane gas if the gas is not reliquefied. LNG must thus be handled cryogenically during storage and transport, i.e. that tanks, pipes, swivels and valves must be thermally isolated. During LNG production at sea this liquid gas must be transferred to an LNG tank vessel which brings the LNG load ashore to separate tank devices arranged for receiving LNG.
Gas condensate consists of other lighter hydrocarbon fractions which must be stored in ordinary tanks separate from the cryogenically stored LNG. Normally the gas condensate must be transported at ordinary tank vessel and may not be transported at LNG tank vessels. Gas condensate may be transferred via e.g. floating loading hoses or STL systems to tank vessels or other export systems. A floating loading hose arranged for being stored or mooraged along the FPSO vessel while not in use for transfer of liquid load, e.g. condensate, is described in Navion""s patent application NO 19980431, filed 30.01.1998.
FIG. 12 illustrates a known solution for LNG production and export which implies storage of a very large volume of LNG on board the FPSO vessel""s LNG tanks, and fast offloading to a tank vessel. The large storage volume contributes to the FPSO vessel""s displacement and implies generally high construction, operating and maintenance costs. The large storage volume demands a volume which could rather have been utilized for other production processes or storage of other petroleum fluids. LNG tank vessels are already available, and they are less specialized than FPSO vessels, and may thus represent a more economic solution for intermittent storage during production of LNG at the field.
Due to the known moorage device""s limitations, and in order to keep safe separation, preferably 100-150 metres between the FPSO vessel and the tank vessel, known LNG pipes must be long. Existing solutions for LNG transfer to LNG tank vessels imply use of stiff transfer pipes typically 16xe2x80x3 (40 cm) inner diameter, and relatively fast transfer to a LNG tank vessel. The known systems for LNG transfer with 16xe2x80x3 pipe implies e.g. loading of 135000 m3 of LNG during about 12 hours, i.e. 10000 m3/hour. Such long transfer pipes arranged in a crane boom are heavy, stiff and difficult to handle and are often damaged by connection or disconnection, or break during transfer. Moorage and load transfer under demanding operating conditions is difficult to perform due to the mechanical loads such a transfer pipe would exerted to.
A mooring system comprising hawsers from the FPSO vessel""s stern to the LNG vessel""s bow in cooperation with about 40 to 50 tonnes constant force aftwards from the tank vessel propulsion engine in order to keep a very close, still tensioned mooring between the FPSO vessel and a tank vessel, is described in Navion""s patent application NO 998 0579 filed 10.02.1998, of which this application claims priority. A support vessel which would otherwise be present anyway for handling of loading hoses, moorings etc., may replace the aftward force from the tank vessel""s propulsion engine. The combination of a close and still tensioned mooring position facilitates transfer of ordinary liquid load, preferably gas condensate, through the floating loading hose to a midship manifold or a bow manifold on an ordinary tank vessel, but also launches the possibility of LNG load transfer via a flexible LNG pipe extended between the stern of the FPSO vessel and the bow of an LNG tanker. This flexible LNG pipe may either hang freely and dry between the vessels, be held e.g. on sheaves by means of a support wire extended between the vessels, or running via the sea.
A need therefore exists of a system being able to handle a continuous production of both a large volume of LNG and a smaller proportion of gas condensate, and which may export these two products with each their different and special demands for storage, handling and load transfer, preferably to two different kinds of vessels, by means of two different load transfer systems.
The system which here is being applied patent for, represents a unitary solution for several of the above mentioned problems of technical, operational and logistic nature.
The solution to the above mentioned problems consists of a system for production, storage and export of liquid natural gas (LNG) from an FPSO vessel with an LNG liquefaction plant, with the novel trait by the invention is the combination of the following points:
an LNG buffer tank on the FPSO vessel, with buffer storage capacity for temporary storage of the continuously produced LNG during the absence of an LNG tank vessel,
a mooring device arranged for short separation moorage between the stern of the FPSO vessel and the bow of an LNG tank vessel,
a cryogenic transfer device arranged between the FPSO vessels stern and an LNG tank vessel""s bow, comprising a flexible LNG pipe and arranged for consecutive transfer of produced LNG, and
at least one or more LNG storage tanks in an LNG tank vessel, arranged for being filled continuously via the cryogenic transfer device until the desired degree of filling of an LNG tank vessel is achieved.
The invention comprises also a method for floating production, storage and export of liquid natural gas (LNG) by means of a system according to claim 1, with the new and inventive is a repeated series of the following steps:
a) moorage of an LNG vessel""s bow to an FPSO vessel""s stern by means of a mooring device arranged for short separation moorage,
b) connection of a cryogenic transfer device, arranged for continuous transfer of produced LNG arranged by the FPSO vessels stern, to an LNG tank vessel""s bow,
c) consecutive transfer of the continuously produced LNG from the LNG liquefaction plant via the cryogenic transfer device (4) to the LNG storage tanks on board an LNG tank vessel, until the desired degree of filling of an LNG tank vessel is achieved,
d) disconnection of an LNG tank vessel, simultaneously with
e) continuous production and temporary storage of the produced LNG in the LNG buffer tank on board the FPSO vessel,
f) moorage of an LNG tank vessel as in point (a) and connection of an LNG tank vessel to the FPSO vessel via the cryogenic transfer device and discharging of the LNG buffer tank to an LNG tank vessel simultaneously with the resumption of consecutive transfer of produced LNG to an LNG tank vessel.
The invention also concerns a method for floating production, storage and export of liquid natural gas (LNG) and gas condensate by means of the system according to claim 16, with the novel and inventive consisting of a repeated sequence of the following steps:
a) moorage of an LNG tanker""s bow to the FPSO vessels stern by means of a moorage device arranged for short separation moorage,
b) connection of a transfer device arranged for transfer of produced LNG arranged by the FPSO vessel""s stern, to an LNG tank vessel""s bow,
c) consecutive transfer of the produced LNG from the LNG liquefaction plant via the cryogenic transfer device to the LNG storage tanks on board an LNG tank vessel, until a desired degree of filling of the LNG storage tanks are achieved,
d) storage of the produced gas condensate in gas condensate tanks on board the FPSO vessel,
e) disconnection of the LNG tank vessel simultaneously with continued consecutive production and temporary production to the LNG buffer tank on board the FPSO vessel and either
f) during an LNG tank vessel""s absence or disconnected state, to connect together an ordinary tank vessel with the FPSO vessel and transfer of the gas condensate via a separate transfer device to tanks in a tank vessel, or
g) moorage of and connection of an LNG tank vessel to the FPSO vessel via the cryogenic transfer device and discharging of the LNG buffer tank to an LNG tank vessel.
The invention also consists as a device for transfer of liquid natural gas (LNG) from an FPSO vessel to a tank vessel by means of a cryogenic transfer device, with the new and inventive is that it comprises
(a) a crane boom arranged by the stern of the FPSO vessel, comprising a relatively rigid LNG pipe, which boom is rotatable about a horizontal axis, with the LNG pipe being connected with
(b) at least one flexible LNG pipe, arranged for connection to
(c) a connector arranged in the bow of the LNG tank vessel.
With a preferred embodiment of the invention one may have smaller FPSO vessels with equally large production capacity as FPSO vessels designed according to the known art.
The system of the invention implies that as utilization of parts of the saved volume in the FPSO vessel by consecutively transferring the storage of the production of LNG to an LNG tank vessel being mooraged the FPSO vessel, larger storage tanks for gas condensate may be arranged in the FPSO vessel, arranged for storing the usually smaller proportion of gas condensate being produced, and arrangements for transfer of the gas condensate to an ordinary tank vessel. The invention comprises a method for transferring the stored gas condensate from the tanks of the FPSO vessel via a floating loading hose to a separate tank vessel arranged for transporting such gas condensate. Preferably this gas condensate is transferred while the LNG tank vessel is absent from the FPSO vessel.
The purpose with such a system, a method and a device as mentioned above and according to the set of claims, is to have an FPSO vessel with small storage volume of LNG, with small moorage separation to an LNG tank vessel, with both lying in tandem with the bow toward the weather. The FPSO vessel has continuous production of LNG and a consecutive and relatively slow transfer of LNG takes place via a cryogenic flexible pipe from the FPSO vessel""s stern to the bow of the LNG tank vessel. The LNG tank vessel acts as a temporary storage for LNG. The LNG transfer is continued until a desired degree of filling of the LNG tank vessel is achieved. The LNG buffer tank on board the FPSO vessel is therefore filled up normally during the short period from the interrupting of the LNG transfer when the LNG tank vessel goes to the shore and deliver at a receiving plant, and until an other LNG tank vessel is back, mooraged and connected to the FPSO vessel again, before the LNG buffer tank becomes full. In the wider perspective opened by the invention, an ordinary tank vessel for condensate may be connected to the FPSO vessel via a floating loading hose, and receive the gas condensate which has been produced during a longer period. During the absence of the LNG tank vessel the continuously produced LNG is stored temporarily in the LNG buffer tank on board the FPSO vessel. When the ordinary tank vessel eventually is disconnected from the floating loading hose and leaves the FPSO vessel, an LNG tank vessel is mooraged for consecutive storage of the LNG production simultaneously with the LNG buffer tank""s content being transferred to the LNG tank vessel. With the system and the method according to the invention the production of LNG and condensate may happen continuously, and both products may be stored and exported in a way which is more to the purpose and economic than according to the known art.
Additional inventive traits are evident from the description and the dependent claims.